Method and system for determining post exposure bake endpoint

ABSTRACT

A method of detecting post exposure bake endpoint during processing of a semiconductor substrate. The method includes providing a radiation source coupled to a post exposure bake station and providing a radiation detector coupled to the post exposure bake station. The method also includes directing a radiation signal generated by the radiation source through an absorption region coupled to the substrate. The method further includes measuring a first detected signal at the radiation detector, measuring a second detected signal at the radiation detector, and comparing the first detected signal and the second detected signal to determine the post exposure bake endpoint.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. application Ser. No. 60/639,109, filed Dec. 22, 2004, entitled “Twin Architecture For Processing A Substrate,” which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of substrate processing equipment. More particularly, the present invention relates to a method and apparatus for monitoring process parameters during a photolithography process. Merely by way of example, the invention has been applied to detecting an endpoint during a post exposure bake process subsequent to photolithographic exposure in a scanner. The method and apparatus can be applied to other processes for semiconductor substrates, for example those used in the formation of integrated circuits.

Modern integrated circuits contain millions of individual elements that are formed by patterning the materials, such as silicon, metal and/or dielectric layers, that make up the integrated circuit to sizes that are small fractions of a micrometer. The technique used throughout the industry for forming such patterns is photolithography. A typical photolithography process sequence generally includes depositing one or more uniform photoresist (resist) layers on the surface of a substrate, drying and curing the deposited layers, patterning the substrate by exposing the photoresist layer to electromagnetic radiation that is suitable for modifying the exposed layer and then developing the patterned photoresist layer.

It is common in the semiconductor industry for many of the steps associated with the photolithography process to be performed in a multi-chamber processing system (e.g., a cluster tool) that has the capability to sequentially process semiconductor wafers in a controlled manner. One example of a cluster tool that is used to deposit (i.e., coat) and develop a photoresist material is commonly referred to as a track lithography tool.

Track lithography tools typically include a mainframe that houses multiple chambers (which are sometimes referred to herein as stations) dedicated to performing the various tasks associated with pre- and post-lithography processing. There are typically both wet and dry processing chambers within track lithography tools. Wet chambers include coat and/or develop bowls, while dry chambers include thermal control units that house bake and/or chill plates. Track lithography tools also frequently include one or more pod/cassette mounting devices, such as an industry standard FOUP (front opening unified pod), to receive substrates from and return substrates to the clean room, multiple substrate transfer robots to transfer substrates between the various chambers/stations of the track tool and an interface that allows the tool to be operatively coupled to a lithography exposure tool in order to transfer substrates into the exposure tool and receive substrates from the exposure tool after the substrates are processed within the exposure tool.

Over the years there has been a strong push within the semiconductor industry to shrink the size of semiconductor devices. The reduced feature sizes have caused the industry's tolerance to process variability to shrink, which in turn, has resulted in semiconductor manufacturing specifications having more stringent requirements for process uniformity and repeatability. An important factor in minimizing process variability during track lithography processing sequences is to ensure that substrates processed within the chambers of the track lithography tool undergo repeatable processing steps. Thus, process engineers will typically monitor and control the device fabrication processes to ensure repeatability from substrate to substrate.

One approach to providing repeatability is to perform processing steps for a predetermined time. However, given that some processing variables change as a function of time (e.g., chamber temperature, substrate temperature, etc.), merely controlling the timing of a process step may not ensure the desired process repeatability. Ultimately, time-varying process parameters may directly affect process variability and ultimately device performance.

In view of these requirements, methods and techniques are needed to ensure process repeatability during semiconductor processing operations using track lithography and other types of cluster tools.

SUMMARY OF THE INVENTION

According to the present invention, methods and apparatus related to semiconductor manufacturing equipment are provided. More particularly, the present invention relates to a method and apparatus for monitoring process parameters during a photolithography process. Merely by way of example, the invention has been applied to detecting an endpoint during a post exposure bake process subsequent to photolithographic exposure in a scanner. While some embodiments of the invention are particularly useful in detecting post exposure bake endpoint in a chamber or station of a track lithography tool, other embodiments of the invention can be used in other applications where it is desirable to manage semiconductor processing operations in a highly controllable manner.

According to an embodiment of the present invention, a method of detecting post exposure bake endpoint during processing of a semiconductor substrate is provided. The method includes providing a radiation source coupled to a post exposure bake station. In some embodiments, the radiation source is an optical radiation source with a wavelength tuned to correspond to an absorption feature associated with a byproduct of the post exposure bake process. Merely by way of example, the radiation source in a particular embodiment is a laser. The method also includes providing a radiation detector coupled to the post exposure bake station. The method further includes directing a radiation signal generated by the radiation source through an absorption region coupled to the substrate. In an embodiment, the absorption region contains reaction byproducts from a chemically amplified resist produced during a portion of a PEB process. The method additionally includes measuring a first detected signal at the radiation detector, measuring a second detected signal at the radiation detector, and comparing the first detected signal and the second detected signal to determine the post exposure bake endpoint.

According to an alternative embodiment of the present invention, an apparatus for detecting post exposure bake endpoint is provided. The apparatus includes a source of optical radiation disposed in a post exposure bake chamber and an optical path coupling an optical radiation detector to the source of optical radiation, wherein the optical path passes through a region coupled to a semiconductor substrate. In an embodiment, the source of optical radiation is a semiconductor laser emitting infrared radiation. In another embodiment, the source of optical radiation is an optical fiber coupled to a laser, the laser provided at a remote location. Moreover, in yet another embodiment, the optical path includes a portion substantially parallel to the a surface of the semiconductor substrate. Merely by way of example, in a particular embodiment, the region coupled to the semiconductor substrate is an absorption region associated with byproducts of the post exposure bake process.

The apparatus also includes a controller coupled to the source of optical radiation and the optical radiation detector, wherein the controller calculates a value associated with an optical absorption coefficient and provides a termination signal in response to the value. In a specific embodiment, the termination signal is provided to a system controller for a track photolithography tool.

According to yet another embodiment of the present invention, a method for detecting post exposure bake endpoint is provided. The method includes providing an optical source coupled to a post exposure bake chamber and providing an optical detector coupled to the optical source through an optical path. In an embodiment, the optical source is tunable. The method also includes exposing a substrate comprising a layer of photoresist to an exposure process. For example, the exposure process in a specific embodiment is a photolithographic exposure process. In some embodiments, the photoresist is a chemically amplified photoresist. The method further includes directing an optical signal from the tunable optical source toward the substrate, directing a reflected optical signal from the substrate to the optical detector, and measuring an interference pattern produced by the optical signal and the reflected optical signal. In a particular embodiment, the optical source and the detector are provided at a location external to the post exposure bake chamber. The method additionally includes generating a control signal to indicate the post exposure bake endpoint.

According to an alternative embodiment of the present invention, a track photolithography tool is provided. The tool includes a PEB station, a source of optical radiation coupled to the PEB station, and an optical radiation detector coupled to the source of optical radiation by an optical path, wherein the optical path passes through a region coupled to a semiconductor substrate undergoing a PEB process. The tool also includes a controller coupled to the source of optical radiation and the optical radiation detector, wherein the controller calculates a value associated with an optical absorption coefficient and provides a termination signal in response to the value.

Many benefits are achieved by way of the present invention over conventional techniques. For example, an embodiment provides a more repeatable post exposure bake process than conventional designs, resulting in improved control over critical dimensions. Moreover, other embodiments of the present invention reduce the post exposure bake processing time by terminating the process upon endpoint detection rather than a predetermined time. Additionally, the methods and apparatus of the present invention minimize over baking of the substrate during the post exposure bake process. Furthermore, in some embodiments, the methods and apparatus of the present invention provide an early indication of upstream or present process variation by observing whether or not the indicated endpoint time is within a normal variation range. Depending upon the embodiment, one or more of these benefits, as well as other benefits, may be achieved. These and other benefits will be described in more detail throughout the present specification and more particularly below in conjunction with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified plan view of an embodiment of a track lithography tool according to an embodiment of the present invention;

FIG. 2 is a simplified schematic side view of a post exposure bake chamber according to an embodiment of the present invention;

FIG. 3 is a simplified flow chart illustrating a process of detecting post exposure bake endpoint according to an embodiment of the present invention;

FIG. 4 is a simplified schematic side view of a post exposure bake chamber according to an alternative embodiment of the present invention; and

FIG. 5 is a simplified flow chart illustrating a process of detecting post exposure bake endpoint according to an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

According to the present invention, methods and apparatus related to semiconductor manufacturing equipment are provided. More particularly, the present invention relates to a method and apparatus for monitoring process parameters during a photolithography process. Merely by way of example, the invention has been applied to detecting an endpoint during a post exposure bake process subsequent to photolithographic exposure in a scanner. While some embodiments of the invention are particularly useful in detecting post exposure bake endpoint in a chamber or station of a track lithography tool, other embodiments of the invention can be used in other applications where it is desirable to manage semiconductor processing operations in a highly controllable manner.

FIG. 1 is a plan view of an embodiment of a track lithography tool 100 in which the embodiments of the present invention may be used. As illustrated in FIG. 1, track lithography tool 100 contains a front end module 110 (sometimes referred to as a factory interface or FI), a central module 112, and a rear module 114 (sometimes referred to as a scanner interface). Front end module 110 generally contains one or more pod assemblies or FOUPS (e.g., items 116A-D), a front end robot 118, and front end processing racks 120A and 120B. The one or more pod assemblies 116A-D are generally adapted to accept one or more cassettes 130 that may contain one or more substrates or wafers, “W,” that are to be processed in track lithography tool 100.

Central module 112 generally contains a first central processing rack 122A, a second central processing rack 122B, and a central robot 124. Rear module 114 generally contains first and second rear processing racks 126A and 126B and a back end robot 128. Front end robot 118 is adapted to access processing modules in front end processing racks 120A, 120B; central robot 124 is adapted to access processing modules in front end processing racks 120A, 120B, first central processing rack 122A, second central processing rack 122B and/or rear processing racks 126A, 126B; and back end robot 128 is adapted to access processing modules in the rear processing racks 126A, 126B and in some cases exchange substrates with a stepper/scanner 5.

The stepper/scanner 5, which may be purchased from Canon USA, Inc. of San Jose, Calif., Nikon Precision Inc. of Belmont, Calif., or ASML US, Inc. of Tempe Ariz., is a lithographic projection apparatus used, for example, in the manufacture of integrated circuits (ICs). The scanner/stepper tool 5 exposes a photosensitive material (resist), deposited on the substrate in the cluster tool, to some form of electromagnetic radiation to generate a circuit pattern corresponding to an individual layer of the integrated circuit (IC) device to be formed on the substrate surface.

Each of the processing racks 120A, 120B; 122A, 122B and 126A, 126B contain multiple processing modules in a vertically stacked arrangement. That is, each of the processing racks may contain multiple stacked integrated thermal units 10, multiple stacked coater modules 132, multiple stacked coater/developer modules with shared dispense 134 or other modules that are adapted to perform the various processing steps required of a track photolithography tool. As examples, coater modules 132 may deposit a bottom antireflective coating (BARC); coater/developer modules 134 may be used to deposit and/or develop photoresist layers and integrated thermal units 10 may perform bake and chill operations associated with hardening BARC and/or photoresist layers.

In one embodiment, a system controller 140 is used to control all of the components and processes performed in the cluster tool 100. The controller 140 is generally adapted to communicate with the stepper/scanner 5, monitor and control aspects of the processes performed in the cluster tool 100, and is adapted to control all aspects of the complete substrate processing sequence. In some instances, controller 140 works in conjunction with other controllers, such as a post exposure bake (PEB) controller as described more fully below, to control certain aspects of the processing sequence. The controller 140, which is typically a microprocessor-based controller, is configured to receive inputs from a user and/or various sensors in one of the processing chambers and appropriately control the processing chamber components in accordance with the various inputs and software instructions retained in the controller's memory. The controller 140 generally contains memory and a CPU (not shown) which are utilized by the controller to retain various programs, process the programs, and execute the programs when necessary. The memory (not shown) is connected to the CPU, and may be one or more of a readily available memory, such as random access memory (RAM), read only memory (ROM), floppy disk, hard disk, or any other form of digital storage, local or remote. Software instructions and data can be coded and stored within the memory for instructing the CPU. The support circuits (not shown) are also connected to the CPU for supporting the processor in a conventional manner. The support circuits may include cache, power supplies, clock circuits, input/output circuitry, subsystems, and the like all well known in the art. A program (or computer instructions) readable by the controller 140 determines which tasks are performable in the processing chamber(s). Preferably, the program is software readable by the controller 140 and includes instructions to monitor and control the process based on defined rules and input data.

It is to be understood that embodiments of the invention are not limited to use with a track lithography tool such as that depicted in FIG. 1. Instead, embodiments of the invention may be used in any track lithography tool including the many different tool configurations described in U.S. application Ser. No. ______ entitled “Twin Architecture for Processing a Substrate” filed on Apr. 21, 2005 (attorney docket number AM 009540), which is hereby incorporated by reference for all purposes and including configurations not described in the AM 009540 application.

In some embodiments of the present invention, chemically amplified photoresists are utilized during the photolithography process. These resists typically include a photosensitive acid generator and a polymer that is acid sensitive. Generally, a chemically amplified resist provides increased sensitivity to exposure in scanner 5, enabling a reduction in exposure dose. When the resist is exposed in the scanner, the photosensitive acid is produced, interacting with the acid sensitive polymer to produce a pattern in the resist. During the PEB process following exposure, the acid produces further reactions. In this way, a single photon may produce many chemical events, thus the reference to these resists as chemically amplified resists.

Moreover, during the PEB process, the photogenerated acid may diffuse in the film, producing undesirable variations in line widths and critical dimensions. Accordingly, embodiments of the present invention provide for monitoring and control of the PEB process, detecting an endpoint, and terminating the PEB process in a repeatable manner.

FIG. 2 is a simplified schematic side view of a post exposure bake chamber according to an embodiment of the present invention. Although the PEB chamber illustrated in FIG. 2 is not integrated with a chill chamber, this is not required by the present invention. In some embodiments, the PEB chamber is an integrated thermal unit as illustrated by reference number 10 in FIG. 1, including, for example, a bake station, a chill station, and a shuttle station. The methods and systems of the present invention are utilized in conjunction with the bake stations of these integrated units. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

During the PEB process, the chemical reactions described above produce chemical byproducts. In a specific embodiment according to the present invention, reaction byproducts in the form of vapors or gases are released by the reaction between the photosensitive acid and the acid sensitive polymer. Merely by way of example, these reaction byproducts may be hydrocarbon containing materials (organics), carbon dioxide (CO₂), and the like. Moreover, in some embodiments, combinations of these reaction byproducts are produced. As the chemical reactions proceed, the concentrations of the various reaction byproducts change as will be evident to one of skill in the art. In some embodiments of the present invention, the reaction byproduct concentrations are monitored as a function of time to determine the PEB endpoint.

In an embodiment of the present invention as illustrated in FIG. 2, a laser 220 emits a beam (see item “A”) at a preselected wavelength. In a specific embodiment, the wavelength is tuned to correlate with an absorption peak of one or more of the reaction byproducts. As illustrated, radiation from the laser passes through a region adjacent the upper surface of the wafer “W.” The concentration of reaction byproducts in the form of vapors or gases will be increased in this region with respect to other portions of the chamber. Thus, an absorption region is coupled to the substrate. In another embodiment, the absorption region is defined by geometry, for example, a geometrical region with a base defined by a surface of the substrate, a height extending to a predetermined distance above the surface of the substrate, and a length approximately equal to the diameter of the substrate. In a specific embodiment, the predetermined distance is about 0.1 cm. In other embodiments, the distance ranges from about 0.02 cm to about 0.5 cm.

As radiation from the laser 220 propagates through the absorption region, the intensity of the signal received by the detector 222 is decreased due to absorption processes in relation to the signal produced by laser 220. Preferably, the wavelength and intensity of the laser is also selected so that the laser will not potentially cause further exposure of the photoresist.

In a particular embodiment, the wavelength, or wavelengths, emitted by the laser are between about 500 nm and about 4000 nm, spanning both the visible and infrared spectrum. In one embodiment, where a concentration of carbon dioxide vapor is being detected, the wavelength of the laser is selected to be about 1960 nm, which corresponds to a peak in the absorption profile of CO₂. As one of skill in the art, this wavelength range is provided by several commercial semiconductor lasers, including InGaAsP diode lasers. In another embodiment, the wavelength of the beam emitted by the laser is 4230 nm, which is produced by mid-infrared semiconductor lasers including quantum cascade lasers.

Reaction byproducts produced during the PEB process diffuse out from the resist layer as well as away from the resist layer. As illustrated in FIG. 2, the optical path “A” is positioned adjacent the surface of the substrate “W” resting on substrate support 212, thereby passing through a region in which the reaction byproducts are present. The intensity of light (I) passing through an absorbing medium is calculated using equation 1: I=I ₀ e ^(−αLC),   [1] wherein I₀ is the initial intensity, a is the absorption coefficient, L is the path length, and C is the species concentration. The product αLC is a unitless quantity. Therefore, a species concentration for a particular absorbing species may be calculated by measuring I and I₀ for a given path length L and absorption coefficient α.

In some embodiments according to an embodiment of the present invention, the species concentration changes as a function of time during the PEB process. Merely by way of example, in a particular photolithography process, the species concentration will initially increase as reaction byproducts are produced during the baking process and subsequently decrease as the reactions proceed to completion. Utilizing embodiments according to the present invention, the species concentration may be mapped as a function of time, enabling a system operator to preferentially select a PEB endpoint.

As illustrated in FIG. 2, the beam emitted by laser 220 passes just above the surface along a path parallel and in close proximity to the photoresist coating the surface of the substrate “W.” The substrate support 212 (sometimes referred to as a plate assembly) may be, for example, a PEB plate assembly, which is used to process the substrate during a bake, PEB, or HMDS process. Since the concentration of the reaction byproducts are the highest just above the surface of the photoresist, the endpoint detection system will generally have the highest sensitivity to changes in the concentration of the byproducts in the gas or vapor phase in this configuration. An additional advantage of this configuration is that by projecting the beam over the surface of the photoresist, the detected variation in intensity is the sum of the amount of byproducts passing through the beam over the whole length of the beam. This method provides a lower signal to noise ratio, and also corrects for variations in the process during different phases of the process.

Endpoint controller 201 is coupled to both the laser source 220 and the detector 222. As illustrated, the endpoint controller receives data from both the laser and detector and communicates data to the laser and detector. For example, in an embodiment, the laser includes built-in power monitoring circuitry, providing an input to the endpoint controller related to the laser power generated by the laser source. Moreover, the endpoint controller may provide numerous inputs to the laser source, including operating current, wavelength, and temperature, among other parameters. Endpoint controller 201 includes computing, memory, and communications functions, enabling the endpoint controller to interact not only with the illustrated PEB chamber, but other chambers as well as the photolithography track system controller 140.

Although FIG. 2 illustrates a straight optical path between the laser source 220 and the detector 222, this straight path is not required by the present invention. As one of skill in the art will appreciate, reflective elements located around the periphery of the substrate may be used to extend the optical path, providing multiple passes over the substrate surface. Moreover, although the laser source 220 is illustrated as contained inside the processing environment 210, this arrangement is not required by the present invention. In alternative embodiments, a laser source located outside the PEB chamber 200 is coupled to the environment 210 through a fiber optic cable. Additionally, as will be evident to one of skill in the art, additional spectroscopic techniques, including the use of a White cell and cavity ring down spectroscopy (CRDS) approaches may be employed in embodiments according to the present invention. In a particular embodiment, a tunable laser source is used to perform Fourier Transform Infrared (FTIR) Spectroscopy, analyzing various absorption features to detect an endpoint for the PEB process.

FIG. 3 is a simplified flow chart illustrating a process of detecting post exposure bake endpoint according to an embodiment of the present invention. Generally, the PEB endpoint detection process is performed for a semiconductor substrate after a photolithographic exposure step. In step 310, a radiation source coupled to a PEB station is provided. In some embodiments, the radiation source is a laser tuned to an absorption feature associated with a byproduct of the PEB process. Additionally, a radiation detector coupled to the PEB station is provided in step 312. In some embodiments, both the radiation source and detector are provided at locations inside the station, whereas in other embodiments, the source and detector are located outside the station and coupled to the station via optical systems. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

A radiation signal generated by the radiation source is directed through an absorption region coupled to a substrate in step 314. Merely by way of example, the absorption region is a region adjacent the surface of the substrate in some embodiments of the present invention. In a particular embodiment, the absorption region contains reaction byproducts from a chemically amplified resist during a portion of a PEB process. Exemplary reaction byproducts are hydrocarbons, alkenes, olephins, combinations of these or the like. In another embodiment, the absorption region is defined by geometry, for example, a geometrical region with a base defined by a surface of the substrate, a height extending to about 0.1 cm above the surface of the substrate, and a length approximately equal to the diameter of the substrate. As described above, radiation propagating through the absorption region experiences differing levels of absorption during different portions of the PEB process.

A first detected signal is measured at the radiation detector in step 316. In an embodiment, the first detected signal is measured at an initial stage of the PEB process, providing a baseline measurement. In other embodiments, the first detected signal is obtained at later stages. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. In step 318, a second detected signal is measured at the radiation detector. Merely by way of example, embodiments of the present invention are not limited to the collection of only two detected signals. In some embodiments, multiple detected signals are acquired as a function of time, with uniform or varying periods between acquisition of detected signals.

The first detected signal and the second detected signal are compared in step 320 to determine the PEB endpoint. In a particular embodiment, multiple signals are collected and analyzed to track the production of reaction byproducts as a function of time. As an example, as the chemically amplified resist reaches a process endpoint, the byproduct levels may drop, enabling a PEB endpoint to be detected, not based on PEB time, but on chemical changes occurring on the substrate. Accordingly, such in-situ measurements provide means to account for variations in processes, exposure levels in the scanner, resist thicknesses, and the like.

FIG. 4 is a simplified schematic side view of a post exposure bake chamber according to an alternative embodiment of the present invention. As illustrated in FIG. 4, a processing chamber 400 is provided according to embodiments of the present invention. In this alternative embodiment of the present invention, an optical system 410 is used to determine the photoresist layer thickness. In yet another alternative embodiment, the optical system 410 is used to sense a change in the index of refraction of the photoresist layer to determine the endpoint of the PEB process. In some embodiments, these embodiments are combined to determine both photoresist layer thickness and sense a change in the index of refraction of the photoresist layer.

The endpoint detection system 410 generally contains a laser 412, a beam splitter 414 and a detector 416. In the embodiment shown in FIG. 4, the endpoint detection system 410 also contains a fiber optic cable 418 which can allow the optical system, including the laser 410, beam splitter 414 and detector 416, to be positioned a predetermined distance from the processing region 420 above the surface of the substrate “W.” The laser operating parameters, including output wavelength and the like, as well as the optical properties of the beamsplitter and detector, including optical coatings, detectivity as a function of wavelength, and the like, are selected to provide a desired level of system performance. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

In a specific embodiment, the laser 412 is operated in a multiple wavelength emission mode, generating multiple wavelengths either sequentially, simultaneously, or combinations thereof. Accordingly, the thickness of a photoresist layer “P” and/or index of refraction changes associated with the photoresist layer “P” may be monitored during the processing. Merely by way of example, the thickness of the photoresist layer is measured in an embodiment by detecting a change in multi-wavelength interference patterns that will change as the photoresist thickness and index of refraction change during the process.

Referring to FIG. 4, in a particular embodiment of the endpoint detection process, the laser 412 emits radiation that is incident on beamsplitter 414. As is well known to one of skill in the art, at the beamsplitter, a percentage of the radiation emitted from the laser 412 passes directly through the beamsplitter 414 and impinges on the fiber optic cable 418. The fiber optic cable 418 then directs the energy passing through the beamsplitter towards the surface of the substrate “W” with a layer of photoresist “P” present on the top surface. The radiation is then reflected, scattered, or absorbed at the surface of the photoresist layer “P” and/or the surface of the substrate. A percentage of the reflected radiation then travels back to the fiber optic 418 where it is directed to the beamsplitter 414. The beamsplitter 414 then reflects a percentage of the reflected radiation to the detector 416 where the radiation incident on the detector is measured.

In some embodiments, in order to detect when an endpoint of a PEB process has occurred, using either of the embodiments described above, the detected signal is compared with a previously detected signal or data collected during the previous processing of substrates. In a particular embodiment, post-PEB process measurements are obtained for a given process and information from these measurements is utilized during the PEB endpoint detection process. As an example, in an embodiment, the thickness of the photoresist layer is measured as a function of time and the PEB endpoint is determined based on the photoresist layer thickness. Utilizing some photoresist layers, chemical shrinkage processes result in photoresist layer thickness changes during the PEB process. Accordingly, measurements of the photoresist layer thickness are a metric utilized to determine PEB endpoint according to some embodiments of the present invention.

FIG. 5 is a simplified flow chart illustrating a process of detecting post exposure bake endpoint according to an alternative embodiment of the present invention. As described above, in some embodiments, the endpoint detection process is optimized by using data collected from previously processed wafers. In step 510, at least one reference endpoint signal is recorded for one or more wafers. In a specific embodiment, a number of reference signals for one or more of wafers numbered I through N are acquired and recorded. These reference signals may be endpoint detection signals acquired using the optical system 410 as illustrated in FIG. 4. In other embodiments, these reference signals may be endpoint detection signals acquired using the PEB endpoint system illustrated in FIG. 2. In a particular embodiment, the one or more reference signals are stored in a memory of a system controller 140 as illustrated in FIG. 1.

After acquisition and recording of the at least one reference signal for wafers 1 to N, post-PEB processing is performed on the wafers or substrates. These processes include development, drying, inspection, and the like in some embodiments. Preferably, inspection steps are performed to determine how the various structural features present on the substrate compared with a design criteria. For example, critical dimensions, linewidths, and the like are compared to determine similarities and differences between the endpoint signals for various substrates and the various structural features. As will be evident to one of skill in the art, correlations between endpoint signals and device features are calculated for use in future PEB endpoint determinations.

In step 514, an endpoint signal for a wafer numbered N+1 is collected. The endpoint signal for wafer N+1 is compared with endpoint signals associated with one or more of the wafers numbered 1 through N in step 516. In step 518, a PEB process time is selected for wafer N+1 based on the comparison step. Utilizing this iterative process, substrate processing operations, and PEB endpoint determination, in particular, are optimized to determine and select preferable PEB processes.

While the present invention has been described with respect to particular embodiments and specific examples thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention. The scope of the invention should, therefore, be determined with reference to the appended claims along with their full scope of equivalents. 

1. A method of detecting post exposure bake endpoint during processing of a semiconductor substrate, the method comprising: providing a radiation source coupled to a post exposure bake station; providing a radiation detector coupled to the post exposure bake station; directing a radiation signal generated by the radiation source through an absorption region coupled to the substrate; measuring a first detected signal at the radiation detector; measuring a second detected signal at the radiation detector; and comparing the first detected signal and the second detected signal to determine the post exposure bake endpoint.
 2. The method of claim 1 wherein the radiation source is an optical radiation source.
 3. The method of claim 2 wherein a wavelength of the optical radiation source is tuned to correspond to an absorption feature associated with a byproduct of the post exposure bake process.
 4. The method of claim 3 wherein the radiation source is a laser.
 5. The method of claim 1 wherein the absorption region contains reaction byproducts from a chemically amplified resist during a portion of a PEB process.
 6. The method of claim 5 wherein the reaction byproducts are hydrocarbons, CO₂, alkenes, and/or olefins.
 7. The method of claim 1 wherein the absorption region is a geometrical region with a base defined by a surface of the substrate and a height extending to about 0.1 cm above the surface of the substrate, and a length approximately equal to the diameter of the substrate.
 8. An apparatus for detecting post exposure bake endpoint, the apparatus comprising: a source of optical radiation disposed in a post exposure bake chamber; an optical path coupling an optical radiation detector to the source of optical radiation, wherein the optical path passes through a region coupled to a substrate support adapted to support a semiconductor substrate; and a controller coupled to the source of optical radiation and the optical radiation detector, wherein the controller calculates a value associated with an optical absorption coefficient and provides a termination signal in response to the value.
 9. The apparatus of claim 8 wherein the source of optical radiation is a laser.
 10. The apparatus of claim 8 wherein the laser is a semiconductor laser emitting infrared radiation.
 11. The apparatus of claim 8 wherein the source of optical radiation is an optical fiber coupled to a laser, the laser provided at a remote location.
 12. The apparatus of claim 8 wherein the optical path includes a portion substantially parallel to the a surface of the semiconductor substrate.
 13. The apparatus of claim 8 wherein the termination signal is provided to a system controller for a track photolithography tool.
 14. The apparatus of claim 8 wherein the region coupled to the semiconductor substrate is an absorption region associated with byproducts of the post exposure bake process.
 15. A method for detecting post exposure bake (PEB) endpoint in a PEB station comprising an optical source coupled to the PEB station and an optical detector coupled to the optical source through an optical path, the method comprising: exposing a substrate comprising a layer of photoresist to an exposure process; directing an optical signal from the tunable optical source toward the substrate; directing a reflected optical signal from the substrate to the optical detector; measuring an interference pattern produced by the optical signal and the reflected optical signal; and generating a control signal to indicate the PEB endpoint.
 16. The method of claim 15 wherein the optical source is tunable.
 17. The method of claim 15 wherein the exposure process is a photolithographic exposure process.
 18. The method of claim 15 wherein the photoresist is a chemically amplified photoresist.
 19. The method of claim 15 wherein the optical source and the detector are provided at a location external to the PEB chamber.
 20. A track photolithography tool, the tool comprising: a PEB station; a source of optical radiation coupled to the PEB station; an optical radiation detector coupled to the source of optical radiation by an optical path, wherein the optical path passes through a region adjacent to a substrate support adapted to support a semiconductor substrate undergoing a PEB process; and a controller coupled to the source of optical radiation and the optical radiation detector, wherein the controller calculates a value associated with an optical absorption coefficient and provides a termination signal in response to the value.
 21. The track lithography tool of claim 20 wherein the source of optical radiation and the optical radiation detector are provided at a location external to the PEB station. 